Crystal Shell : so let me get this straight, if an opposing character hits Crystar on its first click by rolling 18, Crystar doesn't reduce the damage? Or do they mean rolling 18 vs 18 def is "succeeding by exactly 1" The wording is so confusing.

Yes, it was a little confusing. However, it was officially clarified, rolling the exact number means no reducing.
https://win.wizkids.com/bb/viewtopic...6f57b32#p32284

hmm, would this power stack with other reducing defense powers?

No. It would be either this power or another reducer.